There has conventionally been a drill head for deep-hole drilling employing a throwaway tip for a blade body, as shown in FIG. 8(A) and FIG. 8(B), for example. This drill head 50 has a substantially cylindrical head main body 51 with a hollow portion 52 opened at a proximal end side and a head distal end surface 51a of a substantially obtuse conical shape provided with one large and one small cutting chip discharge port 53A, 53B forming a substantially fan shape whose central angle is 90 degrees and communicating with the hollow portion 52. A center cutting blade body 54A and a circumference side blade body 54B are formed on an opening side edge of the large cutting chip discharge port 53A along a head radial direction D, and an intermediate portion blade body 54C is formed on an opening side edge of the small cutting chip discharge port 53B along the same head radial direction D by screw-clamping throwaway tips 60 on recessed mounting seats of the head main body 51 side, respectively. Further, on an outer circumferential surface 51b of the head main body 51, guide pads 55A to 55D are screw-clamped at the distal end side and a male thread 56 is provided at the proximal end side.
In deep-hole drilling work, a proximal portion side of the drill head 50 is threaded into and mounted on a distal end portion of a hollow boring bar (not shown) by the male thread 56, and the boring bar is coupled to a driving shaft such as a spindle of a machine tool to be rotatingly-driven or otherwise rotate a work material, thereby drilling the work material by cutting bodies 54A to 54C to form a deep-hole. Moreover, it is noted that a relative rotation direction of the drill head 50 is a counterclockwise direction in FIG. 8(A). During the deep-hole drilling work, a coolant is supplied into a cutting region through a gap between a cutting hole and the boring bar at high pressure, made to flow from the cutting chip discharge ports 53A, 53B into the hollow portion 52 together with cutting chips generated in the cutting region, and discharged outside through the inside of the hollow boring bar.
The throwaway tip 60 on this occasion is of a hexagonal thick plate body shape, has a three-fold symmetry in which a sectional shape when divided into three sections at the central angle of 120 degrees is equalized, and has an inner blade edge 62 shorter in blade length and an outer blade edge 61 longer in blade length adjacent to each other and constituting one unit of an angular shaped blade portion. That is, each throwaway tip 60 is mounted by a fixing bolt 63 having passed through a bolt insertion hole provided at the center thereof in such a manner that an angular shaped blade portion at one place faces the front end of the head main body as a blade edge. When the angular shaped blade portion reaches its wear limit or gets damaged, the mounting posture is turned 120 degrees and a new angular shaped blade portion can be made to face the front end. As a result, the throwaway tip has an advantage of being usable three times in total by rotating the tip to present a new blade edge and thus having a longer service life. See Japanese Published Unexamined Patent Application No. H07-068408.
On the other hand, this kind of drill head 50 carries out whole cutting so that an entire cutting hole is formed by drilling. Therefore, the center cutting blade body 54A responsible for cutting the central portion of a cutting hole is required to be precisely positioned and mounted in such a manner that a blade edge matches with a diametral line D about a head shaft center axis O and an inner end 62a of the inner blade edge 62 is arranged slightly beyond the head shaft center axis O. Therefore, a cutting rate becomes zero theoretically at the head shaft center axis O during deep-hole drilling work, and thus, a blade edge portion positioned in the shaft center O does not exert a cutting force as what is referred to as a chisel edge, which results in crushing a work material, and thrust resistance is loaded. As a result, this has been a factor that cutting efficiency cannot be enhanced.
Consequently, as for a deep-hole drilling tool employing a throwaway tip with a two-fold symmetry as a blade body, more specifically, a throwaway tip of a parallelogrammatic thick plate body, having blade edges on opposed two sides and being usable twice in total by replacing the blade edge by a 180 degree turn, the present inventor has already proposed one configured such that a sinking portion is provided to a tip side surface at a blade edge inner end side, and the blade edge inner end is arranged spaced apart from the head shaft center axis, thereby forming a non-cutting zone in the vicinity of the shaft center and breaking off an uncut core of a work material generated in the non-cutting zone by press-contacting with an inclined step of the sinking portion. See Japanese Published Unexamined Patent Application No. 2003-25129 & Japanese Published Unexamined Patent Application No. 2003-236713.
However, when the inner end of the blade edge is arranged so as to be spaced apart from the head shaft center axis in the three-fold symmetric throwaway tip 60 as used in the drill head as shown in FIG. 8, there is a concern that the uncut core of the work material generated in the non-cutting zone cannot be broken off as in the afore-described two-fold symmetric throwaway tip and the uncut core grows long, thereby degrading cutting chip dischargeability and additionally causing a cutting chip clogging, which falls into cutting inability. As a result, the three-fold symmetric throwaway tip has been applicable only to the normal whole cutting type.